This invention relates to semiconductor lasers for use as light sources in optical-fiber communication systems.
A semiconductor laser suitable for use in optical-fiber communication systems must have a low threshold current value, stably oscillate in the single transversal mode and so on. Therefore, the generally used semiconductor lasers are constructed to provide, in the direction parallel to the junction and adjacent to a stripe-shaped active layer of double hetero structure, several semiconductor layers including current-blocking layers of which the refractive index is smaller than that of the active layer, for the purpose of confining light and current within the active layer. FIGS. 1A and 1B show typical examples thereof. FIG. 1A is a schematic cross-sectional diagram of a so-called BC-structure semiconductor laser (buried-crescent laser, hereinafter abbreviated BC laser), and FIG. 1B is a schematic cross-sectional diagram of a so-called BH structure semiconductor laser (buried-hetero structure laser, hereinafter abbreviated BH laser).
In FIG. 1A, there are shown an n-type InP substrate 1, an n-type InP layer 2, a B-type InP layer 3, an n-type InP layer 4, an n-type InP clad layer 5, an n-type InGaAsP active layer 6, a p-type InP clad layer 7, a p-type InGaAsP cap layer 8 and ohmic electrodes 9 and 10. The confinement of current within the n-type InGaAsP active layer 6 is carried out by the p-n junction which is reverse-biased by the bias applied to the laser diode of the p-type InP layer 3 and the n-type InP layer 4 formed on the layer 3.
The BC laser, as described, for example, in Electronics Letters, 5th August, 1982 Vol. 18, No. 16, pp. 703-705, has a thyristor structure, p-n-p-n junction formed of the InP layers 2, 3, 4 and 7 which structure is turned on to allow leakage current to flow as indicated by I.sub.L1 in FIG. 1A even when a small leakage current flows in the p-type InP layer 3, forming the gate of the thyrister, at a high temperature. In order to make it difficult to turn on the thyristor, it is necessary to precisely control the thickness of each InP layer 2, 3, 4, 7 and the carrier concentration and therefore the growth process is complicated. Also, as described in Electronics Letters, 26th May, 1983, Vol. 19 No. 11, pp. 407-408, when the p-n junction formed by the p-type InP layer 3 and the n-type InP clad layer 5, lying along the wall of the groove in n-type InGaAsP active layer 6 is buried, is deteriorated, the diffusion potential decreases to a value lower than that of the junction formed n-type InGaAsP active layer 6 and the P-type InP clad layer 7, with the result that the leakage current flows as indicated by I.sub.L2 in FIG. 1A. These leakage currents are relatively large as compared with the current flowing in the n-type InGaAsP active layer 6, thus causing an increase in the threshold current value, saturation in the high output side of the laser output, decrease in the quantum efficiency, and so on.
In FIG. 1B, there are shown an n-type InP substrate 11, an n-type InP clad layer 12, an n-type InGaAsP active layer 13, a p-type InP clad layer 14, a p-type InGaAsP cap layer 15, a p-type InP layer 16, an n-type InP layer 17, and ohmic electrodes 18 and 19. The BH laser has also a thyristor structure, p-n-p-n junction formed by the InP layers 12, 16, 17 and 14 and thus has leakage currents I.sub.L1 and I.sub.L2 as in the case of the BC laser shown in FIG. 1A. The confinement of current within the active layer is generally performed by providing several layers having at least one p-n junction which is reverse-biased by the bias applied to the laser diode, to be adjacent to a stripe-shaped active layer of double hetero structure in the direction parallel to the junction. However, the two or more layers having at least one such p-n junction can not avoid the leakage currents I.sub.L1 and I.sub.L2 due to the structures shown in FIGS. 1A and 1B. The leakage currents are also caused by deterioration of the reverse-biased junction itself and any defect caused in the junction interface upon crystal growth.